Have you plugged in a PC into the ONT and tested yet? I'm struggling to understand the actual issue here.
Can you draw a simple network diagram when the problem occurs?
Sorry I am late getting back to you.
I have the ONT to either third part router or there own (Makes no difference) then either direct to desktop. I have a 100/1Gig switch rack and 2 other switches that are 2.5gbps. the 2.5 switches are for use to my desktops and micro server as they all have 2.5g NIC cards.
Now to rule out my "stuff" I have just gone from the ONT to desktop and I have high speed on Ookla go up to maybe 900Mbps then drop to 500 or so and creap back up to 800. or Go high and finish at 400. This is most often on the DL, although the UL does suffer sometimes.
On Sat I put BRSK router on so they could monitor ir, we could only do it for a short period as some sites would not open during that period (so had to go back to my router which they couldn't monitor due to some settign in the TP Link Deco router I couldn't set for them.
The results they got showed no issues really. - Yet the problem remains.
I have loads and loads of logs from my router but i couldnt make anysense of what i read!?